Pressure-sensing probes are utilized in numerous biomedical and physiological monitoring procedures for measuring pressures prevailing in the body of a subject. Two principal types of pressure monitoring devices are commonly utilized in the biomedical arts. The so-called "extracorporeal" type utilizes a transducer positioned outside of the patient's body and connected to the region of interest by a hollow catheter or the like. The pressure from the area of interest is transmitted by a column of fluid. This approach has significant disadvantages including generally poor response to dynamic, rapidly changing pressures at the tip of the catheter and the practical difficulties associated with maintaining an open, unclogged lumen within the patient's body.
In another approach, commonly referred to as a "intracorporeal" or "in situ" sensing approach, a pressure transducer is physically inserted into the body so that the transducer itself is disposed within the region of interest for direct monitoring of the pressure therein. Typically, the pressure transducer is mounted on an elongated body such as a catheter at the distal end thereof. The pressure transducer typically is connected by wires or other appropriate signal-transmitting means to the proximal end of the probe, i.e., the end of the probe structure which is positioned outside of the subject's body during use. Because the pressure is measured directly at the region of interest, without an intervening fluid column, many of the difficulties associated with extracorporeal pressure sensing are eliminated. However, a pressure transducer for placement within the patient's body ordinarily must be very small. For example, a transducer for insertion through an intravascular catheter typically should be less than about 2.3 mm wide. Moreover, such a transducer should be safe, accurate and manufacturable at reasonable cost. All these requirements taken together pose a formidable engineering challenge.
A desirable transducer for intracorporeal pressure sensing is set forth in commonly assigned U.S. Pat. No. 4,554,927. As set forth in the '927 patent, the pressure transducer may incorporate a piezoresistive beam element positioned inside of a housing adjacent an opening in the housing. An imperforate flexible diaphragm extends across the opening. A plate may be mounted on the interior surface of the flexible diaphragm. Pressure applied on the diaphragm would be transmitted to the plate and hence to the beam element. Accordingly, the beam element would be subjected to a load directly related to the difference between external pressure surrounding the exterior of the housing and a reference pressure inside of the housing. The beam element will provide an electrical resistance directly related to this force and hence directly related to the pressure differential. As further set forth in the '927 patent, this beam element may be formed in conjunction with another element as a unitary, U-shaped element. The second element provides a resistance which varies with temperature and allows compensation for temperature effects on the first-said beam element.
As set forth in commonly assigned U.S. Pat. No. 4,886,070, the flexible diaphragm desirably applies a preload to the sensor. That is, when the external pressure equals the internal pressure, tension in the diaphragm itself desirably applies a force to the force transducer. Such preload is desirable to assure that the diaphragm is effectively linked to the transducer element during normal conditions of use.
Apart from temperature compensation, there is still need to calibrate the sensor and the electronic signal-processing devices used to monitor the signal from the sensor. Typically, the signal processing equipment has both "gain" and "zero" settings. The zero setting adjusts the system to display a reading corresponding to a predetermined baseline value, typically zero, when the probe is surrounded by a predetermined deform pressure, typically atmospheric pressure. Where the reference and datum pressures are both atmospheric pressure, then the datum pressure condition corresponds to a condition of zero differential pressure applied at the transducer, i.e., a condition where the pressure outside and inside the probe housing are the same. The gain setting controls the amount by which the displayed pressure value will change for a given change in the actual pressure surrounding the probe. Both the zero and gain settings ordinarily must be adjusted to correct values for an individual instrument and for an individual electronic signal process system. Moreover, it is often necessary to readjust these settings to compensate for changes in the signal processing equipment, the probe or both during the course of a medical procedure. For example, it may be necessary to readjust these values if the signal processing equipment is momentarily disconnected during the course of a procedure, or if the signal processing equipment is changed and new equipment is substituted. It is difficult to adjust the zero setting of the system while an intracorporeal pressure sensor is in place within the patient. The sensor is exposed to the varying pressure prevailing in the patient's body, and hence cannot be placed under the datum pressure.
Considerable effort has been devoted by the art heretofore towards a solution to this problem. Thus, as shown in U.S. Pat. No. 4,936,310 and 4,712,566, a moveable valve device may be provided for isolating the transducer from the surroundings so that the transducer may be subjected to datum pressure for zero setting purposes. These arrangements add size and complexity to the system. The aforementioned U.S. Pat. No. 4,886,070 discloses a method for achieving calibration, including zero setting, while the probe remains in place. In this method, the interior or reference pressure inside of the pressure sensor is varied in a systematic manner and changes in the displayed readings are observed. One reference point used in this process is the pressure required to overcome the preload applied by diaphragm and also overcome the external pressure prevailing in the vicinity of the pressure sensor. Although this method can achieve accurate calibration while the pressure sensor remains in place, still further simplification would be desirable. Thus, it would be desirable if the user could, through a simple procedure, bring the sensor to a condition where the sensor will provide the same output as it would provide under the reference or datum conditions.
One approach which has been employed heretofore is to surround the entire distal tip of the probe structure with a flexible balloon. In normal use the pressure within the patient's body surrounding the distal tip is exerted on the pressure sensor through the balloon. For example, Gobiet et al, Experience With An Intracranial Pressure Transducer Readjustable In Vivo, Technical Note, J. Neurosurgery, Volume 39, pp. 272-276 (February, 1974) discloses a pressure sensing probe having a diaphragm-equipped pressure transducer at the distal end of the probe structure. The probe structure has interior or reference space at the distal end connected to a port or source of reference pressure through a first lumen. A balloon surrounds the entire distal end of the sensor body, the balloon being connected to another pressure source through a separate second lumen. During normal operation, atmospheric pressure is applied through both lumens. The fluid pressure surrounding the distal tip, being greater than atmospheric pressure forces the balloon inwardly against the diaphragm so that the pressure transducer is subjected to a differential pressure corresponding to the difference between the fluid pressure and the atmospheric pressure prevailing inside the reference space. When an arbitrary pressure greater than the surrounding fluid pressure is applied through both lumen simultaneously, the balloon is inflated and hence stands away from the pressure transducer, thereby isolating the transducer from the surroundings within the patient. Because the pressure inside the reference space is the same as the pressure applied within the balloon, there is zero differential pressure across the diaphragm. The pressure transducer "sees" a zero differential pressure, exactly the same condition as would prevail if the distal tip were exposed to atmospheric pressure while the reference pressure was also atmospheric. Therefore, the zero setting of the signal processing system can be adjusted in this condition.
Van Berg, U.S. Pat. No. 4,901,735 discloses another, similar diaphragm-type pressure transducer which uses only a single lumen for connection of the reference and balloon pressures. Thus, in the '735 patent device, the diaphragm is mounted in a housing at the distal end of the device, the housing having several ports remote from the diaphragm itself leading to the exterior surface of the housing. The entire housing, including the various ports, is encased by the balloon. When a reference pressure applied through the single lumen connected to this housing exceeds the surrounding pressure, the balloon is inflated and pushed away from the diaphragm, thereby placing the diaphragm-type pressure transducer under a zero-differential pressure condition and permitting zero-setting. When the reference pressure applied through this single lumen is less than the surrounding pressure, the balloon collapses and the diaphragm is again exposed to the differential pressure. Both of these approaches impart considerable size and complexity to the pressure sensor, particularly at the distal tip of the sensor body. U.S. Pat. No. 3,703,099 discloses other, similar devices. Although the principal disclosure in the '099 patent is directed to pressure transducers other than diaphragm-type transducers, the '099 patent does mention that one could use a diaphragm-type transducer with the distensible membrane or balloon system and with "fluid access means", not further defined, for both sides of the diaphragm.
Despite all of this effort in the prior art however, there is still need for further improvement. In particular, there is a need for a device which would combine the accurate and convenient zero-setting capabilities of the isolation balloon or membrane-type devices with the compactness and the other desirable attributes of diaphragm-type pressure sensors such as those shown in the '927 and '070 patents.